Photon neutralizers for neutral beam injectors

ABSTRACT

A non-resonance photo-neutralizer for negative ion-based neutral beam injectors. The non-resonance photo-neutralizer utilizes a nonresonant photon accumulation, wherein the path of a photon becomes tangled and trapped in a certain space region, i.e., the photon trap. The trap is preferably formed by two smooth mirror surfaces facing each other with at least one of the mirrors being concave. In its simplest form, the trap is elliptical. A confinement region is a region near a family of normals, which are common to both mirror surfaces. The photons with a sufficiently small angle of deviation from the nearest common normal are confined. Depending on specific conditions, the shape of the mirror surface may be one of spherical, elliptical, cylindrical, or toroidal geometry, or a combination thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject application is a continuation of PCT Patent Application No.PCT/US15/61356, filed Nov. 18, 2015, which claims priority to RussianPatent Application No. 2014146574, filed on Nov. 19, 2014, both of whichare incorporated by reference herein in their entirety for all purposes.

FIELD

The subject matter described herein relates generally to neutral beaminjectors and, more particularly, to a photon neutralizer for a neutralbeam injector based on negative ions.

BACKGROUND

A traditional approach to produce a neutral beam from a negative ion H−,D− beam for plasma heating or neutral beam assisted diagnostics, is toneutralize the negative ion beam in a gas or plasma target fordetachment of the excess electrons. However, this approach has asignificant limitation on efficiency. At present, for example, fordesigned heating injectors with a 1 MeV beam [R. Hemsworth et al., 2009,Nucl. Fusion 49 045006], the neutralization efficiency in the gas andplasma targets will be about 60% and 85%, respectively [G. I. Dimov etal., 1975, Nucl. Fusion 15, 551], which considerably affects the overallefficiency of the injectors. In addition, the application of suchneutralizers is associated with complications, including thedeterioration of vacuum conditions due to gas puffing and the appearanceof positive ions in the atomic beam, which can be significant in someapplications.

Photodetachment of an electron from high-energy negative ions is anattractive method of beam neutralization. Such method does not require agas or plasma puffing into the neutralizer vessel, it does not producepositive ions, and it assists with beam cleaning of fractions ofimpurities due to negative ions. The photodetachment of an electroncorresponds to the following process: H⁻+hω=H⁰+e. Similar to mostnegative ions, the H− ion has a single stable state. Nevertheless,photodetachment is possible from an excited state. The photodetachmentcross section is well known [see, e.g., L. M.Branscomb et al., Phys.Rev. Lett. 98, 1028 (1955)]. The photodetachment cross section is largeenough in a broad photon energy range which practically overlaps allvisible and near IR spectrums.

Such photons cannot knock out an electron from H0 or all electrons fromH− and produce positive ions. This approach was proposed in 1975 by J.H. Fink and A. M. Frank [J. H. Fink et al., Photodetachment of electronsfrom negative ions in a 200 keV deuterium beam source, LawrenceLivermore Natl. Lab. (1975), UCRL-16844]. Since that time a number ofprojects for photon neutralizers have been proposed. As a rule, thephoton neutralizer projects have been based on an optic resonatorsimilar to Fabri-Perot cells. Such an optic resonator needs mirrors withvery high reflectance and a powerful light source with a thin line, andall of the optic elements need to be tuned very precisely. For example,in a scheme considered by Kovari [M. Kovari et al., Fusion Engineeringand Design 85 (2010) 745-751], the reflectance of the mirrors isrequired to be not less than 99.96%, the total laser output power isrequired to be about 800 kW with output intensity of about 300 W/cm²,and the laser bandwidth is required to be less than 100 Hz. It isunlikely that such parameters could be realized together.

Therefore, it is desirable to provide a non-resonance photo-neutralizer.

SUMMARY OF INVENTION

Embodiments provided herein are directed to systems and methods for anon-resonance photo-neutralizer for negative ion-based neutral beaminjectors. The non-resonance photo-neutralizer described herein is basedon the principle of nonresonant photon accumulation, wherein the path ofthe photon becomes tangled and trapped in a certain space region, i.e.,the photon trap. The trap is preferably formed as two smooth mirrorsurfaces facing each other with at least one surface being concave. Inthe simplest form, the trap is preferably elliptical in shape. Aconfinement region of the trap is a region near a family of normals thatare common to both mirror surfaces of the trap. The photons with asufficiently small angle of deviation from the nearest common normal areconfined. Depending on specific conditions, the shape of the trap may beone of spherical, elliptical, cylindrical, toroidal, or a combinationthereof.

In operation, photon beams with a given angular spread along and acrossthe trap are injected through one or more small holes in one or more ofthe mirrors. The photon beams can be from standard industrial powerfiber lasers. The photo neutralizer does not require high quality laserradiation sources pumping a photon target, nor does it require very highprecision adjustment and alignment of the optic elements

Other systems, methods, features and advantages of the exampleembodiments will be or will become apparent to one with skill in the artupon examination of the following figures and detailed description.

BRIEF DESCRIPTION OF FIGURES

The details of the example embodiments, including structure andoperation, may be gleaned in part by study of the accompanying figures,in which like reference numerals refer to like parts. The components inthe figures are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention. Moreover, allillustrations are intended to convey concepts, where relative sizes,shapes and other detailed attributes may be illustrated schematicallyrather than literally or precisely.

FIG. 1 is a schematic of a non-resonance photon trap.

FIG. 2 is a schematic of a quasiplanar adiabatic optical trap.

FIG. 3 is a perspective view schematic of the quasiplanar adiabaticoptical trap shown in FIG. 2.

FIG. 4 is a trace of a single ray in the photon trap with a random anglefrom −3° to 5° in the XY plane, and −5° to 5° along the trap, the numberof reflections is 4000. The cone angle of end mirrors is about 3°.

FIG. 5 illustrates an example of the surface intensity distribution andits cross profile in the middle of the trap.

FIG. 6 is a chart showing the degree of neutralization (dotted line) andoverall neutralizer efficiency (continuous curve) vs laser injectionpower.

FIG. 7 is a plan view of a negative ion-based neutral beam injectorlayout.

FIG. 8 is a sectional isometric view of the negative ion-based neutralbeam injector shown in FIG. 7.

It should be noted that elements of similar structures or functions aregenerally represented by like reference numerals for illustrativepurpose throughout the figures. It should also be noted that the figuresare only intended to facilitate the description of the preferredembodiments.

DETAILED DESCRIPTION

Each of the additional features and teachings disclosed below can beutilized separately or in conjunction with other features and teachingsto provide a non-resonance photo-neutralizer for negative ion-basedneutral beam injectors. Representative examples of the embodimentsdescribed herein, which examples utilize many of these additionalfeatures and teachings both separately and in combination, will now bedescribed in further detail with reference to the attached drawings.This detailed description is merely intended to teach a person of skillin the art further details for practicing preferred aspects of thepresent teachings and is not intended to limit the scope of theinvention. Therefore, combinations of features and steps disclosed inthe following detail description may not be necessary to practice theinvention in the broadest sense, and are instead taught merely toparticularly describe representative examples of the present teachings.

Moreover, the various features of the representative examples and thedependent claims may be combined in ways that are not specifically andexplicitly enumerated in order to provide additional useful embodimentsof the present teachings. In addition, it is expressly noted that allfeatures disclosed in the description and/or the claims are intended tobe disclosed separately and independently from each other for thepurpose of original disclosure, as well as for the purpose ofrestricting the claimed subject matter independent of the compositionsof the features in the embodiments and/or the claims. It is alsoexpressly noted that all value ranges or indications of groups ofentities disclose every possible intermediate value or intermediateentity for the purpose of original disclosure, as well as for thepurpose of restricting the claimed subject matter.

Embodiments provided herein are directed to a new non-resonancephoto-neutralizer for negative ion-based neutral beam injectors. Adetailed discussion of a negative ion-based neutral beam injector isprovided in Russian Patent Application No. 2012137795 and PCTApplication No. PCT/US2013/058093, which are incorporated herein byreference.

The non-resonance photo-neutralizer described herein is based on theprinciple of nonresonant photon accumulation, wherein the path of thephoton becomes tangled and trapped in a certain space region, i.e., thephoton trap. The trap is preferably formed as two smooth mirror surfacesfacing each other with at least one surface being concave. In thesimplest form, the trap is preferably elliptical in shape. A confinementregion of the trap is a region near a family of normals that are commonto both mirror surfaces of the trap. The photons with a sufficientlysmall angle of deviation from the nearest common normal are confined.Depending on specific conditions, the shape of the trap may be one ofspherical, elliptical, cylindrical, toroidal, or a combination thereof.

In operation, photon beams with a given angular spread along and acrossthe trap are injected through one or more small holes in one or more ofthe mirrors. The photon beams can be from standard industrial powerfiber lasers. The photo neutralizer does not require high quality laserradiation sources pumping a photon target, nor does it require very highprecision adjustment and alignment of the optic elements.

Turning to the figures, an embodiment of a non-resonance photon trap 10is shown in FIG. 1. As depicted in a two-dimensional case, the trap 10comprises a bottom flat mirror 20 and a top concave mirror 30. A photonγ with a small angle to vertical axes within the trap 10, will developwith each reflection from the upper mirror 30 some horizontal momentumdifference to central axes of trap 10. The position of the photon γafter an n-th reflection is defined by the abscissa of a reflectionpoint, x_(n), with a height, F(x_(n)), an angle φ from a vertical and aphoton speed, β_(n). The horizontal motion is described by the followingsystem of equations:

$\begin{matrix}{{x_{n + 1} - x_{n}} = {\left( {{F\left( x_{n + 1} \right)} + {F\left( x_{n} \right)}} \right){tg}\;\beta_{n}}} & (1) \\{{\beta_{n + 1} - \beta_{n}} = {2\frac{{dF}\left( x_{n + 1} \right)}{dx}}} & (2)\end{matrix}$

For stability investigation, linearize versions of equations (1)and (2)are combined and the following equations are obtained:

$\begin{matrix}{{x_{n + 1} - x_{n}} = {2\;{F(0)}\beta_{n}}} & (3) \\{{\beta_{n + 1} - \beta_{n}} = {2\frac{d^{2}{F(0)}}{{dx}^{2}}x_{n + 1}}} & (4)\end{matrix}$

By combining equations (3) and (4), the following linear recurrencerelation is obtained:

$\begin{matrix}\begin{matrix}{{x_{n + 2} - {2\; x_{n + 1}} + x_{n}} = {4\;{F(0)}\frac{d^{2}{F(0)}}{{dx}^{2}}x_{n + 1}}} \\{{= {{- 4}(0)\frac{x_{n + 1}}{R}}},}\end{matrix} & (5)\end{matrix}$where R is the curvature radius of top mirror 30. Equation (5) is a typeof finite-difference scheme for an oscillation system with unit timestep and with Eigen frequency

$\omega_{0} = {2{\sqrt{\frac{F(0)}{R}}.}}$The solution is representable in the form x_(n)=A·q^(n), where q is acomplex number. Then for q defined as:

$\begin{matrix}{{q_{1,2} = {1 - {\frac{2\;{F(0)}}{R} \pm \sqrt{\left( {1 - \frac{2\;{F(0)}}{R}} \right)^{2} - 1}}}},} & (6)\end{matrix}$The stability condition is |q|≤1, from which photons confinement in ageometric optic, when taking into account non-negativity of value

$\frac{F(0)}{R},$is determined asF(0)<R, ω ₀ ²<4  (7)The curvature radius of the upper mirror 30 impacts photon confinement.Recurrent systems (1) and (2) allow the production of the integral ofmotion:

$\begin{matrix}{{{\sum\limits_{n}\;{{tg}\;{\beta_{n}\left( {\beta_{n + 1} - \beta_{n}} \right)}}} = {\sum\limits_{n}\;{\frac{2\left( {x_{n + 1} - x_{n}} \right)}{{F\left( x_{n + 1} \right)} + {F\left( x_{n} \right)}}\frac{{dF}\left( x_{n + 1} \right)}{dx}}}},} & (8)\end{matrix}$In the case of a sufficiently small curvature of the upper mirror 30 andsmall steps, such as

$\begin{matrix}{{{\Delta\; F} ⪡ F},{\frac{dF}{dx} ⪡ 1},{{\Delta\beta} ⪡ 1},} & (9)\end{matrix}$the integral sums (8) is approximately transformed into

${\ln\frac{\cos\;\beta_{0}}{\cos\;\beta}} = {\ln\frac{F(x)}{F\left( x_{0} \right)}}$or into standard adiabatic invariantF(x)cos(β)=const  (10)Relation (10) determines the region filled by photons.

These estimations enable the design of an effective photon neutralizerfor negative ion beams. Turning to FIGS. 2 and 3, a reasonablethree-dimensional geometry of the trap 10 is a long arch assembly offour components. As depicted in FIG. 2, the trap 10 preferably comprisesa bottom or lower mirror 20 at the bottom of the trap 10 that is planaror flat in shape, and an upper mirror assembly 30 comprising a centralmirror 32 that is cylindrical in shape, and a pair of outer mirrors 34that are conical in shape and coupled to the ends of the central mirror32. As shown, an ion beam H⁻ is passed along the photon trap. The sizesare taken from the characteristic scales of a single neutralizer channelof a beam injector for the International Thermonuclear ExperimentalReactor (ITER).

The following provides results of a numerical simulation of a photonneutralizer for ITER NBI. This simulation has been carried out by usingZEMAX code. FIG. 4 shows a one ray trace in the trap system 10 given inFIG. 2 with a random angle from −3° to 3° in the XY plane, and −5° to 5°along the trap 10.

The trajectory presented in FIG. 4 contains 4000 reflections, afterwhich the ray remained in the trap system. In a resonance device [MKovari, B. Crowley. Fusion Eng. Des. 2010, v. 85 p. 745-751], thestorage efficiency under a mirror reflectance r²=0.9996 is aboutP/P_(in)≈500. In the case noted herein, with a lower mirror reflectanceof r²=0.999, the determined storage efficiency is

$\begin{matrix}{\frac{P}{P_{in}} \approx \frac{1}{1 - r^{2}} \approx 1000} & (11)\end{matrix}$

Losses will tend to be associated chiefly with a large number ofsurfaces inside the cavity and diffraction. [J. H. Fink, Production andNeutralization of Negative Ions and Beams: 3rd Int. Symposium,Brookhaven 1983, AIP, New York, 1984, pp. 547-560]

The distribution of the radiant energy flux through a horizontal planeinside the trap 10 is shown in FIG. 5, where the reflection coefficientof all surfaces is equal to 0.999 and the input radiant power is equalto 1 W. The calculated accumulated power in the cavity of the trap 10 isequal to 722 watts. Taking into account calculation losses (Zemax codemonitors and evaluates such losses) the accumulated power value shouldbe increased by 248 watts. Therefore, the storing efficiency reachesalmost a maximum possible value (11). Thus, quasi-planar systems allowwithin the geometrical optics the creation of a confinement region witha given size.

Note, that the end cone mirrors 34 and main cylindrical mirrors 32 and20 form broken surface as shown in FIGS. 2 and 3. The broken surfacestend to have a negative effect on the longitudinal confinement ofphotons because this forms an instability region (see (7)). However, thenumber of crossings of these borders by a ray during the photon lifetimeis not large in comparison with the total number of reflections, and,thus, the photon does not have time to significantly increaselongitudinal angle and leave the trap through the ends of the trap 10.

Radiation Injection into Trap and Sources

To pump the optic cell, photons beams with a given angular spread alongand across the trap 10 can be injected through one or more small holesin one or more mirrors. For example, it is possible by using a ytterbiumfiber laser (γ=1070 nm, total power above 50 kW)[http://www.ipgphotonics.com/Collateral/Documents/English-US/HP_Brochure.pdf].These serial lasers have sufficient power and their emission line isnear optimal.

The radiation beam with necessary angular spread can be prepared fromfiber laser radiation by special adiabatic conical or parabolic shapers.For example, radiation with a spread of 15° from fiber and Ø300μ may betransformed to 5° and Ø1 mm, which is sufficient for the neutralizertrap 10 described herein.

Efficiency of Photon Neutralization

The degree of neutralization is representable as

$\begin{matrix}{{K(P)} = {1 - {\exp\left( \frac{\sigma\; P}{E_{0}{dV}} \right)}}} & (12)\end{matrix}$where d is the width of the neutralization region, E₀ is the photonenergy, V is the velocity of the ions. P is the total accumulated powerdefined as

${P = \frac{P_{0}}{1 - r^{2}}},$where P₀ is the optic pumping power. The neutralization efficiency of D−flux by the laser with overall efficiency η_(l) may be determined as

$\begin{matrix}{{\eta\left( P_{0} \right)} = \frac{{K(P)}P_{-}}{P_{-} + \frac{P_{0}}{\eta_{l}}}} & (13)\end{matrix}$where P_ is the negative ion beam power. The efficiency increases withgrowth of D− beam power. The efficiency (13) and degree ofneutralization (12) are shown in FIG. 6. This curve has been calculatedfor a single channel gas neutralizer in ITER injectors, in which 10 MWpart is passed. Thus, in such an approach nearly 100% neutralization canbe achieved with very high energetic efficiency of about 90%. Forcomparison, ITER neutral beam injector has a 58% neutralization [R.Hemsworth et al.// Nucl. Fusion. 2009, v. 49, 045006] andcorrespondently the same efficiency. The overall injector efficiencywhile taking into account accelerator supply and transport losses hasbeen estimated by Krylov [A. Krylov, R. S. Hemsworth. Fusion Eng. Des.2006, v.81, p. 2239-2248].

A preferred arrangement of an example embodiment of a negative ion-basedneutral beam injector 100 is illustrated in FIGS. 7 and 8. As depicted,the injector 100 includes an ion source 110, a gate valve 120,deflecting magnets 130 for deflecting a low energy beam line, aninsulator-support 140, a high energy accelerator 150, a gate valve 160,a neutralizer tube (shown schematically) 170, a separating magnet (shownschematically) 180, a gate valve 190, pumping panels 200 and 202, avacuum tank 210 (which is part of a vacuum vessel 250 discussed below),cryosorption pumps 220, and a triplet of quadrupole lenses 230. Theinjector 100, as noted, comprises an ion source 110, an accelerator 150and a neutralizer 170 to produce about a 5 MW neutral beam with energyof about 0.50 to 1.0 MeV. The ion source 110 is located inside thevacuum tank 210 and produces a 9 A negative ion beam. The vacuum tank210 is biased to −880 kV which is relative to ground and installed oninsulating supports 140 inside a larger diameter tank 240 filled withSF6 gas. The ions produced by the ion source are pre-accelerated to 120keV before injection into the high-energy accelerator 150 by anelectrostatic multi aperture grid pre-accelerator 111 in the ion source110, which is used to extract ion beams from the plasma and accelerateto some fraction of the required beam energy. The 120 keV beam from theion source 110 passes through a pair of deflecting magnets 130, whichenable the beam to shift off axis before entering the high energyaccelerator 150. The pumping panels 202 shown between the deflectingmagnets 130 include a partition and cesium trap.

A more detailed discussion of the negative ion-based neutral beaminjector is provided in Russian Patent Application No. 2012137795 andPCT application No. PCT/US2013/058093, which are incorporated herein byreference.

The example embodiments provided herein, however, are merely intended asillustrative examples and not to be limiting in any way.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Forexample, the reader is to understand that the specific ordering andcombination of process actions shown in the process flow diagramsdescribed herein is merely illustrative, unless otherwise stated, andthe invention can be performed using different or additional processactions, or a different combination or ordering of process actions. Asanother example, each feature of one embodiment can be mixed and matchedwith other features shown in other embodiments. Features and processesknown to those of ordinary skill may similarly be incorporated asdesired. Additionally and obviously, features may be added or subtractedas desired. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

What is claimed is:
 1. A non-resonance photo-neutralizer for neutralbeam injectors comprising first and second mirrors having opposingmirror surfaces forming a photon trap, wherein the mirror surface of thefirst mirror is concave and the mirror surface of the second mirror isflat, wherein the first mirror comprises a mirror assembly including acentral mirror and first and second outer mirrors coupled to the centralmirror.
 2. The photo-neutralizer of claim 1 wherein the photon trapcomprises a confinement region adjacent a family of normals common tothe mirror surfaces of the first and second mirrors.
 3. Thephoto-neutralizer of claim 1 wherein the central mirror is cylindricallyshaped and the outer mirrors are conically shaped.
 4. A negative ionbased neutral beam injector comprising a negative ion source, and anon-resonance photo-neutralizer co-axially positioned with the negativeion source, wherein the photo-neutralizer including first and secondmirrors having opposing mirror surfaces forming a photon trap, whereinthe mirror surface of the first mirror is concave and the mirror surfaceof the second mirror is flat, wherein the first mirror comprises amirror assembly including a central mirror and first and second outermirrors coupled to the central mirror.
 5. The neutral beam injector ofclaim 4 wherein the photon trap comprises a confinement region adjacenta family of normals common to the first and second mirror surfaces. 6.The neutral beam injector of claim 4 wherein the central mirror iscylindrically shaped and the outer mirrors are conically shaped.